Selection of ensemble averaging weights for a pulse oximeter based on signal quality metrics

ABSTRACT

A method and a system for ensemble averaging signals in a pulse oximeter, including receiving first and second electromagnetic radiation signals from a blood perfused tissue portion corresponding to two different wavelengths of light, obtaining an assessment of the signal quality of the electromagnetic signals, selecting weights for an ensemble averager using the assessment of signal quality, and ensemble averaging the electromagnetic signals using the ensemble averager.

BACKGROUND OF THE INVENTION

The present invention relates in general to oximeters, and in particularto the selection of ensemble averaging weights used for ensembleaveraging of signals that include detected waveforms from a pulseoximeter.

A pulse oximeter is typically used to measure various bloodcharacteristics including the blood oxygen saturation of hemoglobin inarterial blood and the pulse rate of the patient. Measurement of thesecharacteristics has been accomplished by use of a non-invasive sensorthat passes light through a portion of a patient's blood perfused tissueand photo-electrically senses the absorption and scattering of light insuch tissue. The amount of light absorbed and scattered is then used toestimate the amount of blood constituent in the tissue using variousalgorithms known in the art. The “pulse” in pulse oximetry comes fromthe time varying amount of arterial blood in the tissue during a cardiaccycle. The signal processed from the sensed optical measurement is thefamiliar plethysmographic waveform, which corresponds with the cyclicattenuation of optical energy through a portion of a patient's bloodperfused tissue.

Ensemble averaging, which is a temporal averaging scheme, involves theuse of weighting factors. In a pulse oximeter, ensemble averaging isused to calculate a weighted average of new samples and previousensemble-averaged samples from one pulse-period earlier. Weightsselected and/or used for ensemble averaging have a significant effect onthe ensemble averaging process. Such weights may be uniformly selected,or they may be based on the characteristics of the signals that arebeing ensemble averaged. For example, the Conlon U.S. Pat. No. 4,690,126discloses ensemble averaging where different weights are assigned todifferent pulses and a composite, averaged pulse waveform is used tocalculate oxygen saturation. Conlon's signal metrics for adjustingensemble-averaging weights are based on a measure of the degree ofmotion artifact, a measure of the degree of low perfusion (e.g., pulseamplitude below a threshold), and pulse rate.

However, it is desirable to provide a more flexible and more robustmethodology for the selection of ensemble averaging weights used forensemble averaging of signals that include detected waveforms from apulse oximeter.

BRIEF SUMMARY OF THE INVENTION

The present invention is directed to the selection of ensemble averagingweights used for ensemble averaging of signals that correspond withdetected waveforms from a pulse oximeter. The selection of ensembleaveraging weights are based on one or more or a combination of varioussignal quality metrics or indicators. In one embodiment, the presentinvention provides a method of ensemble averaging signals in a pulseoximeter. The method includes receiving first and second electromagneticradiation signals from a blood perfused tissue portion corresponding totwo different wavelengths of light; obtaining an assessment of thesignal quality of the electromagnetic signals; selecting weights for anensemble averager using the assessment of signal quality; and ensembleaveraging the electromagnetic signals using the ensemble averager.

In one aspect, the selection of ensemble averaging weights involves anassessment and use of various signal quality indicators, where theselecting of weights includes forming a combination of one or more ofthe following signal quality parameters, namely: a measure of the degreeof arrhythmia of the signals; a measure of the degree of similarity orcorrelation between the first and second electromagnetic radiationsignals; a measure of the degree of motion artifact by obtaining a ratioof a current pulse amplitude to the long-term average pulse amplitude ofthe signals; a ratio of a current pulse amplitude to the previous pulseamplitude of the signal; and a ratio of a current pulse period to thatof an average pulse period of the signals.

For a fuller understanding of the nature and advantages of theembodiments of the present invention, reference should be made to thefollowing detailed description taken in conjunction with theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of an exemplary oximeter.

FIG. 2 is a block diagram of the signal processing architecture of apulse oximeter in accordance with one embodiment of the presentinvention.

DETAILED DESCRIPTION OF THE INVENTION

The methods and systems in accordance with the embodiments of thepresent invention are directed towards the selection of ensembleaveraging weights used for ensemble averaging of signals that correspondwith detected waveforms from a pulse oximeter. The selection of ensembleaveraging weights are based on one or more or a combination of varioussignal quality metrics or indicators. The embodiments of the presentinvention are particularly applicable to and will be explained byreference to measurements of oxygen saturation of hemoglobin in arterialblood and pulse or heart rate, as in pulse oximeter monitors and pulseoximetry sensors. However, it should be realized that the embodiments ofthe present invention are equally applicable to any generalized patientmonitor and associated patient sensor, such as ECG, blood pressure,etc., and are thus also applicable to non oximetry or pulse oximetrydevices.

A typical pulse oximeter measures two physiological parameters, percentoxygen saturation of arterial blood hemoglobin (SpO₂ or sat) and pulserate. Oxygen saturation can be estimated using various techniques. Inone common technique, the photocurrent generated by the photo-detectoris conditioned and processed to determine the ratio of modulation ratios(ratio of ratios) of the red to infrared (IR) signals. This modulationratio has been observed to correlate well to arterial oxygen saturation.Pulse oximeters and sensors may be empirically calibrated by measuringthe modulation ratio over a range of in vivo measured arterial oxygensaturations (SaO₂) on a set of patients, healthy volunteers, or animals.The observed correlation is used in an inverse manner to estimate bloodoxygen saturation (SpO₂) based on the measured value of modulationratios of a patient. The estimation of oxygen saturation usingmodulation ratios is described in U.S. Pat. No. 5,853,364, entitled“METHOD AND APPARATUS FOR ESTIMATING PHYSIOLOGICAL PARAMETERS USINGMODEL-BASED ADAPTIVE FILTERING,” issued Dec. 29, 1998, and U.S. Pat. No.4,911,167, entitled “METHOD AND APPARATUS FOR DETECTING OPTICAL PULSES,”issued Mar. 27, 1990, which are both herein incorporated by reference intheir entirety for all purposes. The relationship between oxygensaturation and modulation ratio is described, for example, in U.S. Pat.No. 5,645,059, entitled “MEDICAL SENSOR WITH MODULATED ENCODING SCHEME,”issued Jul. 8, 1997, which is herein incorporated by reference in itsentirety for all purposes. Most pulse oximeters extract theplethysmographic signal having first determined saturation or pulserate, both of which are susceptible to interference.

FIG. 1 is a block diagram of one embodiment of a pulse oximeter that maybe configured to implement the embodiments of the present invention. Theembodiments of the present invention may be implemented as a dataprocessing algorithm that is executed by the microprocessor 122,described below. Light from light source 110 passes into a bloodperfused tissue 112, and is scattered and detected by photodetector 114.A sensor 100 containing the light source and photodetector may alsocontain an encoder 116 which provides signals indicative of thewavelength of light source 110 to allow the oximeter to selectappropriate calibration coefficients for calculating oxygen saturation.Encoder 116 may, for instance, be a resistor.

Sensor 100 is connected to a pulse oximeter 120. The oximeter includes amicroprocessor 122 connected to an internal bus 124. Also connected tothe bus is a RAM memory 126 and a display 128. A time processing unit(TPU) 130 provides timing control signals to light drive circuitry 132which controls when light source 110 is illuminated, and if multiplelight sources are used, the multiplexed timing for the different lightsources. TPU 130 also controls the gating-in of signals fromphotodetector 114 through an amplifier 133 and a switching circuit 134.These signals are sampled at the proper time, depending upon which ofmultiple light sources is illuminated, if multiple light sources areused. The received signal is passed through an amplifier 136, a low passfilter 138, and an analog-to-digital converter 140. The digital data isthen stored in a queued serial module (QSM) 142, for later downloadingto RAM 126 as QSM 142 fills up. In one embodiment, there may be multipleparallel paths of separate amplifier, filter and A/D converters formultiple light wavelengths or spectra received.

Based on the value of the received signals corresponding to the lightreceived by photodetector 114, microprocessor 122 will calculate theoxygen saturation using various algorithms. These algorithms requirecoefficients, which may be empirically determined, corresponding to, forexample, the wavelengths of light used. These are stored in a ROM 146.In a two-wavelength system, the particular set of coefficients chosenfor any pair of wavelength spectra is determined by the value indicatedby encoder 116 corresponding to a particular light source in aparticular sensor 100. In one embodiment, multiple resistor values maybe assigned to select different sets of coefficients. In anotherembodiment, the same resistors are used to select from among thecoefficients appropriate for an infrared source paired with either anear red source or far red source. The selection between whether thenear red or far red set will be chosen can be selected with a controlinput from control inputs 154. Control inputs 154 may be, for instance,a switch on the pulse oximeter, a keyboard, or a port providinginstructions from a remote host computer. Furthermore, any number ofmethods or algorithms may be used to determine a patient's pulse rate,oxygen saturation or any other desired physiological parameter.

The brief description of an exemplary pulse oximeter set forth above,serves as a basis for describing the methods for adjusting ensembleaveraging weights for an ensemble averager, which are described below,in conjunction with FIG. 2.

The embodiments of the present invention may be implemented as a part ofa larger signal processing system used to process optical signals forthe purposes of operating a pulse oximeter. Such a signal processingsystem is shown in FIG. 2, which is a block diagram 200 of a signalprocessing architecture of a pulse oximeter in accordance with oneembodiment of the present invention. The signal processing architecture200 in accordance with the embodiments of the present invention may beimplemented as a software algorithm that is executed by a processor of apulse oximeter. In addition to calculating oxygen saturation and pulserate, the system 200 measures various signal metrics that are used todetermine filter weighting coefficients. Signal metrics are things thatindicate if a pulse is likely a plethysmograph or noise. Signal metricsmay be related to, for example, frequency (is it in the range of a humanheart rate), shape (is it shaped like a cardiac pulse), rise time, etc.The system shown in FIG. 2 calculates both the oxygen saturation, andthe pulse rate. The system 200 is also used for detecting venouspulsation and sensor off and lost pulse conditions, which are describedseparately below.

I. Oxygen Saturation Calculation

Block 202 represents the operation of the Signal Conditioning block. Thedigitized red and IR signals or waveforms are received and areconditioned in this block by: (1) taking the 1^(st) derivative to getrid of baseline shift, (2) low pass filtering with fixed coefficients,and (3) dividing by a DC value to preserve the ratio. The function ofthe Signal Conditioning subsystem is to emphasize the higher frequenciesthat occur in the human plethysmograph and to attenuate low frequenciesin which motion artifact is usually concentrated. The SignalConditioning subsystem selects its filter coefficients (wide or narrowband) based on hardware characteristics identified duringinitialization. Inputs to block 202 are digitized red and IR signals,and its outputs are pre-processed red and IR signals.

Block 204 represents the operation of the Pulse Identification andQualification block. The low pass filtered digitized red and IR signalsare provided to this block to identify pulses, and qualify them aslikely arterial pulses. This is done using a pre-trained neural net, andis primarily done on the IR signal. The pulse is identified by examiningits amplitude, shape and frequency. An input to this block is theaverage pulse period from block 208. This function changes the upfrontqualification using the pulse rate. The output of block 204 indicatesthe degree of arrhythmia and individual pulse quality. Inputs to block204 are: (1) pre-processed red and IR signals, (2) Average pulse period,and (3) lowpass waveforms from the low pass filter. Outputs from block204 include: (1) degree of arrhythmia, (2) pulse amplitude variations,(3) individual pulse quality, (4) pulse beep notification, and (5)qualified pulse periods and age.

Block 206 is used to compute signal quality metrics. This block (block206) determines the pulse shape (e.g., derivative skew), periodvariability, pulse amplitude and variability, Ratio of Ratiosvariability, and frequency content relative to pulse rate. Inputs toblock 206 include: (1) raw digitized red and IR signals, (2) degree ofarrhythmia, individual pulse quality, pulse amplitude variation, (3)pre-processed red and IR signals, and (4) average pulse period. Outputsfrom block 206 include: (1) lowpass and ensemble averaging filterweights, (2) metrics for sensor off detector, (3) normalizedpre-processed waveforms, and (4) percent modulation.

Block 208 computes average pulse periods. This block (block 208)calculates the average pulse period from the pulses received. Inputs toblock 208 include: qualified pulse periods and age. An output from block208 includes the average pulse period.

Block 210 represents the functioning of the lowpass filter and ensembleaveraging subsystem. Block 210 low pass filters and ensemble averagesnormalized and preprocessed waveforms processed by block 206. Theweights for the low pass filter are determined by the Signal Metricsblock 206. The signal is also ensemble averaged (i.e., frequencies otherthan those of interest near the pulse rate and its harmonics areattenuated), with the ensemble averaging filter weights also determinedby Signal Metrics block 206. Less weight is assigned if the signal isflagged as degraded. More weight is assigned if the signal is flagged asarrhythmic because ensemble-averaging is not appropriate duringarrhythmia. Red and IR waveforms are processed separately, but with thesame filtering weights. The filtering is delayed (e.g., approximatelyone second) to allow the signal metrics to be calculated first.

The filters use continuously variable weights. If samples are not to beensemble-averaged, then the weighting for the previous filtered samplesis set to zero in the weighted average, and the new samples are stillprocessed through the algorithm. This block tracks the age of the signaland/or the accumulated amount of filtering (e.g., sum of response timesand delays in processing). Too old a result will be flagged, if goodpulses haven't been detected for a while. The inputs to block 210include: (1) normalized pre-processed red and IR signals, (2) averagepulse period, (3) low pass filter weights and ensemble averaging filterweights, (4) ECG triggers, if available, and (5) IR fundamental, forzero-crossing triggers. Outputs from block 210 include: (1) filtered redand IR signals, and (2) age.

Block 212 represents operations that estimate the ratio-of-ratiosvariance for the filtered waveforms and calculate averaging weights. Thevariable weighting for the filter is controlled by the ratio-of-ratiosvariance. The effect of this variable-weight filtering is that theratio-of-ratios changes slowly as artifact increases and changes quicklyas artifact decreases. The subsystem has two response modes, includingfast and normal modes. For example, filtering in the fast mode targetsan age metric of 3 seconds, and the target age may be 5 seconds in thenormal mode. In the fast mode, the minimum weighting of the currentvalue is clipped at a higher level. In other words, a low weight isassigned to the newest ratio-of-ratios calculation if there is noisepresent, and a high weight if no noise is present. The inputs to block212 include: (1) filtered red and IR signals and age, (2) calibrationcoefficients, and (3) response mode (e.g., user speed settings). Outputsfrom block 212 include an averaging weight for ratio-of-ratioscalculation. The averaging weight is used as an input to block 214 alongwith filtered IR and Red waveforms to calculate averaged ratio of ratiosand age.

Block 216 represents operations that calculate oxygen saturation.Saturation is calculated using an algorithm with the calibrationcoefficients and averaged ratio of ratios. Inputs to block 116 include:(1) Averaged Ratio-of-Ratios, and (2) calibration coefficients. Anoutput from block 216 is the oxygen saturation value.

II. Pulse Rate Calculation

Block 218 low pass filters and ensemble averages the signal(s)conditioned by block 202, for the pulse rate identification. The weightsfor the low pass filter are determined by the Signal Metrics block 206.The signal is also ensemble averaged (i.e., frequencies other than thoseof interest near the pulse rate and its harmonics are attenuated), withthe ensemble averaging filter weights also determined by Signal Metricsblock 206. Less weight is assigned if the signal is flagged as degraded.More weight is assigned if the signal is flagged as arrhythmic becauseensemble-averaging is not appropriate during arrhythmia. Red and IR areprocessed separately. The filtering is delayed (e.g., approximately onesecond) to allow the signal metrics to be calculated first.

The filters use continuously variable weights. If samples are not to beensemble-averaged, then the weighting for the previous filtered samplesis set to zero in the weighted average, and the new samples are stillprocessed through the algorithm. This block (block 218) tracks the ageof the signal and/or the accumulated amount of filtering (sum ofresponse times and delays in processing). Too old a result will beflagged (if good pulses haven't been detected for awhile). Inputs toblock 218 include: (1) pre-processed red and IR signals, (2) averagepulse period, (3) lowpass filter weights and ensemble averaging filterweights, (4) ECG triggers, if available, and (5) IR fundamental, forzero-crossing triggers. Outputs from block 218 include: (1) filtered redand IR signals and (2) age.

Block 220, or the Filtered Pulse Identification and Qualification block,calculates the pulse periods from the filtered waveforms, and itsresults are used only when a pulse is disqualified by block 204. Inputsto block 220 include: (1) filtered red and IR signals and age, (2)average pulse period, (3) hardware ID or noise floor, (4) and the kindor type of sensor that is used to detect the IR and Red energies. Outputfrom block 220 includes qualified pulse periods and age.

Block 222, or the Average Pulse Periods and Calculate Pulse Rate block,calculates the pulse rate and average pulse period. This block (block222) receives qualified pulse periods and age as inputs and provides:(1) average pulse period and (2) pulse rate as outputs.

III. Venous Pulsation

Block 224, or the Detect Venous Pulsation block receives as inputs thepre-processed red and IR signals and age from Block 202, and pulse rateand provides an indication of venous pulsation as an output. Block 224also provides an IR fundamental waveform in the time domain using asingle-tooth comb filter which is output to the Ensemble Averagingfilters (e.g., block 210 and 218). Inputs to block 224 include: (1)filtered red and IR signals and age and (2) pulse rate. Outputs fromblock 124 include: an indication of venous pulsation and IR fundamental.In one embodiment, block 224 measures the “openness” of an IR-RedLissajous plot to determine the whether a flag (e.g., Venous_Pulsation)should be set. The output flag (e.g., Venous_Pulsation) is updatedperiodically (e.g., every second). In addition, the IR fundamentalwaveform is output to the Ensemble Averaging filters.

IV. Sensor Off

Block 226, or the Detect Sensor-Off and Loss of Pulse Amplitude block,uses a pre-trained neural net to determine whether the sensor is off thesurface of the blood-perfused tissue, for example, of a patient. Theinputs to the neural net are metrics that quantify several aspects ofthe behavior of the IR and Red values over the last several seconds.Samples are ignored by many of the system 200's subsystems while thesignal state is either not indicative of a pulse being present, orindicative that a sensor is not on a monitoring site (e.g., PulsePresent, Disconnect, Pulse Lost, Sensor May be Off, and Sensor Off).Inputs to block 226 include: (1) signal quality metrics, and (2) theoximeter's LED brightness, amplifier gain, and (3) an ID indicating theoximeter's hardware configuration. Outputs from block 226 include asignal state including sensor-off indication.

In the architecture 200 described above, the function of block 226,Pulse lost and Pulse Search indications, may be derived usinginformation from several parts of the signal processing architecture. Inaddition, the signal processing architecture will not use the receivedIR and red waveforms to compute oxygen saturation or pulse rate if avalid sensor is not connected, or if the Sensor-Off or Loss of PulseAmplitude are detected by the signal processing architecture.

The brief description of an embodiment of a pulse oximeter signalprocessing architecture in accordance with the present invention, setforth above, serves as a basis for describing the methods and devicesthat are directed towards the selection of ensemble averaging weightsused for ensemble averaging of signals that correspond with detectedwaveforms from a pulse oximeter, as is generally indicated by blocks 210and 218 above.

Ensemble Averaging Weights

As set forth above, the selection of ensemble averaging weights arebased on one or more or a combination of various signal quality metricsor indicators. In particular, in one embodiment, the metrics that areused to adjust ensemble-averaging weight, includes a measure of thedegree of arrhythmia. This metric is used to reduce the degree ofensemble-averaging when the patient appears to be arrhythmic, asensemble-averaging works less well for signals having highly variablefrequency content. Another metric used to adjust ensemble-averagingweight includes a measure of the degree of variability ofratio-of-ratios (e.g., lack of similarity or correlation between IR andRed waveforms). This metric is sensitive to the presence of motion orother noise sources. This metric is different from that of other knowntechniques such as Conlon's, in that Conlon teaches a metric thatcompares the similarity between current and previous-pulse waveforms,presumably from the same wavelength, but not the similarity between twosimultaneous waveforms at different wavelengths. Another metric used toadjust ensemble-averaging weight includes a ratio of a current pulseamplitude to the long-term average pulse amplitude. A long-term averagepulse amplitude refers to an average that has a response time of perhapsa minute when all pulses are qualified, and several times longer if mostpulses are being disqualified. This metric is designed to capture thedegree of motion artifact, similar to Conlon's, however, this metric isan analog metric, whereas Conlon's metric has just a few discrete states(e.g., no artifact, low artifact, high artifact). Another metric used toadjust ensemble-averaging weight includes a ratio of a current pulseamplitude to the previous pulse amplitude. This metric is used toquickly change the ensemble-averaging weight when large motion artifactstart or stop. Another metric used to adjust ensemble-averaging weightincludes a measure of the overall signal quality metric for a singlepulse, which metric is itself a combination of several other metrics,including the metrics described above. This metric is used to quicklyreduce the ensemble filtering when motion artifact subsides and theinput waveform is presumed to be of better quality than a heavilyensemble-averaged waveform. Another metric used to adjustensemble-averaging weight includes a ratio of a current pulse period tothe average pulse period. This metric is used to reduce the ensemblefiltering in the event that the heart skips a beat, which can happenoccasionally on many people.

When the subsystem (210 and/or 218) is notified that the PulseIdentification and Qualification subsystem (204) has just completedevaluation of a potential pulse, the subsystem updatesensemble-averaging weights, used by the instances of the EnsembleAveraging subsystem. Separate weights are computed for the two EnsembleAveraging instances whose outputs are used in computing saturation andpulse rate. These weights are based in part on metrics provided by theinstance of the Pulse Identification and Qualification subsystem whoseinput waveforms have not been ensemble averaged.

The equations for Sat_Ensemble_Averaging_Filter_Weight are as follows:x=max(Short_RoR_Variance,Pulse_Qual_RoR_Variance/1.5)*max(Long_Term_Pulse_Amp_Ratio, 1.0)RoR_Variance_Based_Filt_(—) Wt=0.5*0.05/max(0.05, x)Arr_Prob=(Period_Var−0.1*Short_RoR_Variance−0.09)/(0.25−0.09);Arr_Min_Filt_(—) Wt_For_Sat=0.05+0.5*bound(Arr_Prob, 0, 1.0)Sat_Ensemble_Averaging_Filter_Weight=max(RoR_Variance_Based_Filt_(—) Wt,Arr_Min_Filt_(—) Wt_For_Sat)*(1.0+Pulse_Qual_Score)Sat_Ensemble_Averaging_Filter_Weight=min(Sat_Ensemble_Averaging_Filter_Weight,1.0),where bound(a,b,c) denotes min(max(a,b),c)

The above equations result in a default weight of 0.5 for low values ofthe Ratio-of-Ratios variances. Short_RoR_Variance andPulse_Qual_RoR_Variance are both computed over a time interval (e.g., athree-second interval). The interval for Pulse_Qual_RoR_Variance endswith the qualification or rejection of the most recent pulse, whichwould usually include the most recent samples. The weight is reduced byhigh Ratio-of-Ratios variances, and by high values ofLong_Term_Pulse_Amp_Ratio that would typically indicate motion artifact.Arr_Min_Filt_Wt_For_Sat imposes a minimum value on theensemble-averaging weight (range 0.05–0.55) based primarily onPeriod_Var, which quantifies the degree of arrhythmia. This is donebecause ensemble-averaging is less effective for pulses havingdissimilar periods. If the most recent pulse received a goodPulse_Qual_Score, this can increase the maximum value ofSat_Ensemble_Averaging_Filter_Weight from 0.5 to 1.0.

The equations for Rate_Ensemble_Averaging_Filter_Weight are as follows:Arr_Prob=(Period_Var−0.07)/(0.20−0.07)Arr_Min_Filt_Wt_For_Rate=0.05+0.5*bound(Arr_Prob, 0, 1.0)x=max(RoR_Variance_Based_Filt_(—) Wt, Arr_Min_Filt_(—)Wt_For_Rate)*(1.0+Pulse_Qual_Score)if Short_Term_Pulse_Amp_Ratio*Long_Term_Pulse_Amp_Ratio<1.0x=x/Short_Term_Pulse_Amp_Ratioif Avg_Period>0x=x*bound(Pulse_Qual_Score*Qualified_Pulse_Period/Avg_Period, 1.0, 3.0)Rate_Ensemble_Averaging_Filter_Weight=min(x, 1.0)

These equations differ from the ones forSat_Ensemble_Averaging_Filter_Weight as follows:

-   a) The thresholds used to compute Arr_Prob are somewhat lower,    because it is desirable that arrhythmic pulses not be obscured by    ensemble averaging prior to pulse qualification.-   b) Small values of Short_Term_Pulse_Amp_Ratio typically indicate    that motion artifact has just subsided, which means that the    ensemble-averaging weight may be quickly increased. This has been    found empirically to be beneficial for pulse qualification, but not    for ratio-of-ratios filtering and saturation computation.-   c) If the heart skips a beat, with or without prior arrhythmia, the    longer-than-average Qualified_Pulse_Period that results will    increase the ensemble-averaging weight, so as not to obscure the    skipped beat from subsequent pulse qualification.

In one aspect, the ensemble averaging weights that have been determinedas described above, are used for two separate ensemble averagers forprocessing a detected waveform for use in calculating oxygen saturationand a pulse rate. The ensemble averager used for calculating oxygensaturation operates on a signal which has been normalized, while theensemble averager for the pulse rate calculation operates on a signalwhich has not been normalized. A pulse oximeter with separate ensembleaveraging for oxygen saturation and heart rate is described in patentapplication Ser. No. 11/412,483 assigned to the assignee herein, andtitled: Pulse Oxiemter with Separate Ensemble Averaging for OxygenSaturation and Heart Rate, is hereby incorporated herein by reference,in its entirety for all purposes. In that patent application, themetrics chosen for the two paths through the two ensemble averagers canbe varied to optimize the ensemble averaging for oxygen saturation orpulse rate calculations. For example, a lower threshold is used for ametric to detect arrhythmic pulses when used to calculate pulse rate, ascompared to calculating oxygen saturation. Also, a metric for a shortterm pulse amplitude ratio will be small when motion artifact has justsubsided, and this is given more weight in the pulse rate calculationthan in the oxygen saturation calculation.

DEFINITIONS

Data Inputs

Avg_Period—Average pulse period reported by Pulse Rate Calculationsubsystem.

Long_Term_Pulse_Amp_Ratio—Quantifies last pulse amplitude compared tohistoric pulse amplitude. Provided by the Pulse Identification andQualification subsystem. Values substantially larger than 1.0 aretypically indicative of motion artifact, and result in lowerEnsemble_Averaging_Filter_Weights.

Period_Var—Period-variability metric from the Pulse Identification andQualification subsystem. Used to gauge the extent of arrhythmia. Forinstance, a value of 0.10 would indicate that the average differencebetween consecutive pulse periods is 10% of Avg_Period.

Pulse_Qual_RoR_Variance—RoR_Variance metric from the PulseIdentification and Qualification subsystem.

Pulse_Qual_Score—Score computed by the pulse qualification neural net inthe Pulse Identification and Qualification subsystem. Zero is extremelypoor and 1.0 is excellent.

Qualified_Pulse_Period—Most recent pulse period qualified by the PulseIdentification and Qualification subsystem.

Short_Term_Pulse_Amp_Ratio—Quantifies last pulse amplitude compared toprevious pulse amplitude.

Outputs

Frequency_Ratio—Ratio of Mean_IR_Frequency_Content to pulse rate.

LPF_RoR_Variance—Quantifies variability of ratio-of-ratios. Computedover a 9-second window from LPF_Scaled_Waveforms.

Rate_LPF_Weight—Lowpass filter weight to be used by the instance of theEnsemble Averaging subsystem that preprocesses waveforms used for pulsequalification and pulse rate calculation.

RoR_Variance—Quantifies variability of ratio-of-ratios. Computed over a9-second window from Scaled_Waveforms. For example, a value of 0.10would indicate that sample-to-sample ratio-of-ratios values differ fromthe mean ratio-of-ratios value by an average of 10% of the meanratio-of-ratios value.

Sat_Ensemble_Averaging_Filter_Weight—Ensemble-averaging weight to beused by the instance of the Ensemble Averaging subsystem thatpreprocesses waveforms used for pulse qualification and pulse ratecalculation.

Sat_LPF_Weight—Lowpass filter weight to be used by the instance of theEnsemble Averaging subsystem that preprocesses waveforms used for pulsequalification and pulse rate calculation.

Scaled_Waveforms—Scaled versions of IR and Red Pre_Processed_Waveforms.

Short_RoR_Variance—uantifies variability of ratio-of-ratios. Computedover a 3-second window from Scaled_Waveforms.

Internal Variables

Arr_Prob—Likelihood of arrhythmia that would limit the amount ofensemble averaging. Based on Period_Var, with threshold that arespecific to each of the two Ensemble_Averaging_Filter_Weights.

Arr_Min_Filt_Wt_For_Rate, Arr_Min_Filt_Wt_For_Sat—Minimum values for thetwo Ensemble_Averaging_Filter_Weights, based on their respectiveArr_Prob values.

LPF_Scaled_Waveforms—Lowpass-filtered version of Scaled_Waveforms, usedto compute LPF_RoR_Variance.

Mean_IR_Frequency_Content—Estimate of mean frequency content of the IRinput waveform. Used to compute Frequency_Ratio metric.

RoR_Variance_Based_Filt_Wt—Component forEnsemble_Averaging_Filter_Weights based on RoR_Variance metrics andLong_Term_Pulse_Amp_Ratio.

Accordingly, as will be understood by those of skill in the art, thepresent invention which is related to the selection of ensembleaveraging weights, may be embodied in other specific forms withoutdeparting from the essential characteristics thereof. For example, whilethe present embodiments have been described in the time-domain,frequency-based methods are equally relevant to the embodiments of thepresent invention. Accordingly, the foregoing disclosure is intended tobe illustrative, but not limiting, of the scope of the invention, whichis set forth in the following claims.

1. A method of ensemble averaging signals in a pulse oximeter,comprising: receiving first and second electromagnetic radiation signalsfrom a blood perfused tissue portion corresponding to two differentwavelengths of light; obtaining an assessment of the signal quality ofsaid electromagnetic signals; calculating weights for an ensembleaverager from a continuous weighting function using said assessment ofsignal quality; and ensemble averaging said electromagnetic signalsusing said ensemble averager.
 2. The method of claim 1 wherein saidobtaining an assessment of said signal quality comprises obtaining ameasure of the degree of arrhythmia of said signals.
 3. The method ofclaim 2 wherein said obtaining an assessment of said signal qualityfurther comprises obtaining a measure of the degree of similarity orcorrelation between said first and second electromagnetic radiationsignals.
 4. The method of claim 1 wherein said obtaining an assessmentof said signal quality comprises obtaining a measure of the degree ofmotion artifact present in said signals.
 5. The method of claim 4wherein said obtaining a measure of the degree of motion artifactcomprises obtaining a ratio of a current pulse amplitude to thelong-term average pulse amplitude of said signals.
 6. The method ofclaim 1 wherein said obtaining an assessment of said signal qualitycomprises obtaining a ratio of a current pulse amplitude to the previouspulse amplitude of said signal.
 7. The method of claim 1 wherein saidobtaining an assessment of said signal quality comprises obtaining ameasure of the degree of the overall signal quality metric for a singlepulse, which metric is itself a combination of several other metrics. 8.The method of claim 1 wherein said obtaining an assessment of saidsignal quality comprises obtaining a ratio of a current pulse period tothat of an average pulse period of said signals.
 9. The method of claim1 wherein said calculating weights comprises forming a combination ofone or more parameters selected from the group consisting of a measureof the degree of arrhythmia of said signals, a measure of the degree ofsimilarity or correlation between said first and second electromagneticradiation signals, a measure of the degree of motion artifact byobtaining a ratio of a current pulse amplitude to the long-term averagepulse amplitude of said signals, a ratio of a current pulse amplitude tothe previous pulse amplitude of said signal, and a ratio of a currentpulse period to that of an average pulse period of said signals.
 10. Adevice for ensemble averaging signals in a pulse oximeter, comprising:means for receiving first and second electromagnetic radiation signalsfrom a blood perfused tissue portion corresponding to two differentwavelengths of light; means for obtaining an assessment of the signalquality of said electromagnetic signals; means for calculating weightsfor an ensemble averager from a continuous weighting function using saidassessment of signal quality; and an ensemble averager for ensembleaveraging said electromagnetic signals using said weights.
 11. Thedevice of claim 10 wherein said means for obtaining an assessment ofsaid signal quality are configured for obtaining a measure of the degreeof arrhythmia of said signals.
 12. The device of claim 11 wherein saidmeans for obtaining an assessment of said signal quality are furtherconfigured for obtaining a measure of the degree of similarity orcorrelation between said first and second electromagnetic radiationsignals.
 13. The device of claim 10 wherein said means for obtaining anassessment of said signal quality are configured for obtaining a measureof the degree of motion artifact present in said signals.
 14. The deviceof claim 10 wherein said means for obtaining an assessment of saidsignal quality are configured for obtaining a ratio of a current pulseamplitude to the long-term average pulse amplitude of said signals. 15.The device of claim 10 wherein said means for obtaining an assessment ofsaid signal quality are configured for obtaining a ratio of a currentpulse amplitude to the previous pulse amplitude of said signal.
 16. Thedevice of claim 10 wherein said means for obtaining an assessment ofsaid signal quality are configured for obtaining a measure of the degreeof the overall signal quality metric for a single pulse, which metric isitself a combination of several other metrics.
 17. The device of claim10 wherein said means for obtaining an assessment of said signal qualityare configured for obtaining a ratio of a current pulse period to thatof an average pulse period of said signals.
 18. The device of claim 10wherein said means for selecting weights are configured for forming acombination of one or more parameters selected from the group consistingof a measure of the degree of arrhythmia of said signals, a measure ofthe degree of similarity or correlation between said first and secondelectromagnetic radiation signals, a measure of the degree of motionartifact by obtaining a ratio of a current pulse amplitude to thelong-term average pulse amplitude of said signals, a ratio of a currentpulse amplitude to the previous pulse amplitude of said signal, and aratio of a current pulse period to that of an average pulse period ofsaid signals.
 19. A pulse oximetry system comprising: a sensorconfigured to receive first and second electromagnetic radiation signalsfrom a blood perfused tissue portion corresponding to two differentwavelengths of light; and a pulse oximeter configured to: obtain anassessment of the signal quality of the electromagnetic signals;calculate at least one weight for an ensemble average using acontinuously variable weighting function using the assessment of signalquality; and ensemble average the electromagnetic signals using thecalculated weight.
 20. The pulse oximetry system of claim 19 wherein thepulse oximeter is configured to obtain the assessment of the signalquality by determining a correlation between the first and secondelectromagnetic radiation signals.
 21. The pulse oximetry system ofclaim 19 wherein the pulse oximeter is configured to obtain theassessment of the signal quality by measuring a degree of variability ofa ratio-of-ratios of the first and second electromagnetic radiationsignals.
 22. The pulse oximetry system of claim 19, wherein the pulseoximeter is configured to calculate the at least one weight independentof an ECG signal.
 23. The pulse oximetry system of claim 19, wherein thepulse oximeter is configured to calculate the at least one weight basedon a variability of a ratio-of-ratios over a time.
 24. The pulseoximetry system of claim 19, wherein the pulse oximeter is configured tocalculate the at least one weight based on a pulse qualification score.25. The pulse oximetry system of claim 19, wherein the pulse oximeter isconfigured to calculate the at least one weight based on a pulsequalification score computed by pulse qualification neural net.
 26. Atangible machine readable medium comprising: code adapted to calculate asignal quality metric based on a correlation between a first receivedlight signal and a second received light signal, wherein the firstsignal and the second signal are received at substantially the sametime; code adapted to determine ensemble averaging weights based on acontinuously variable weighting function; and code adapted to ensembleaverage the electromagnetic signals using the determined ensembleaveraging weights.
 27. The tangible machine readable medium of claim 26,wherein the code adapted to calculate the signal quality metric isconfigured to calculate the signal quality based on a ratio of a currentpulse amplitude to a previously-measured pulse amplitude.